Synthesis gases may be formed by steam reforming, partial oxidation, and/or a combination thereof. Thus, pre-reforming and autothermal reforming or primary steam reforming and autothermal or secondary reforming may be used to generate synthesis gases suitable for the production of methanol, dimethyl ether, hydrogen, and hydrocarbons by the Fischer-Tropsch reaction.
The synthesis gases recovered from the reforming apparatus may be cooled before downstream processing using various techniques. In one method, the hot secondary reformed gas mixture is passed through the shell side of a heat exchange reformer containing a plurality of catalyst filled tubes to provide the heat for the primary reforming step. The resulting partially cooled secondary reformed gas mixture may be subjected to one or more further stages of heat exchange. Alternatively the hot, reformed gas mixture may be fed to a waste heat boiler and then used to generate superheated steam before being cooled further in stages of heat exchange. Such heat exchange apparatus typically is fabricated using alloys that comprises metals such as Ni, Cr, and Fe, which under the conditions present in the apparatus, are able to interact with carbon monoxide in the synthesis gas to produce undesirable side reactions including methanation, water-gas shift, and the corrosive carburization reactions, which give rise to so-called “metal dusting.” Whereas higher grade alloys may be used to reduce this problem, these can be costly to use in large reformers. Lower grade alloys may be used if their surfaces are passivated. Passivation of the metal surfaces in heat exchange equipment has been performed in an attempt to prevent the undesirable reactions from taking place.
WO 2007/049069 describes a method for passivating low-alloy steel surfaces in apparatus operating in the temperature range 350 to 580° C. and exposed to a carbon monoxide containing gas mixture comprising adding a passivating compound containing at least one phosphorus (P) atom to said gas mixture.
WO 03/051771 describes a method for reducing the interaction between carbon monoxide present in a heat exchange medium and metal surfaces on the shell side of heat exchange reformer apparatus used for producing a primary reformed gas by treatment of the shell-side of said apparatus with an effective amount of at least one passivation compound containing at least one atom selected from phosphorus, tin, antimony, arsenic, lead, bismuth, copper, germanium, silver, or gold.
Whereas the phosphorus compounds tested were effective in reducing the interaction of carbon monoxide with the alloy surfaces, there is a need to improve the passivation at higher temperatures and under more aggressive synthesis gas compositions.